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Materials 
1. Your palms only 



will observe that the palms have warmed 
up. 




Students to enquire 

1. Why do the palms get warm when 
rubbed together ? 

2. What is the primary source of 
heat energy ? 




What to do ? 

1. Rub your palms a number of times. You 


1. When the palms are rubbed there 
is friction between the palms, and 
the molecules in the palms start 
vibrating faster. Owing to this 
heat is generated and the tem- 
perature increases. 

2. Muscle energy makes the palm 
move. Muscle energy is obtained 
through the intake of food. Plants 
prepare food through photosyn- 
thesis. Hence, solar energy is the primary 
source of heat energy. 



Quest 48 




Materials 

1. Two corks 

2. 10cm. aluminium wire-1 

3. 4cm. aluminium wire-2 

4. Candle and matches 


What to do ? 



1. Insert the aluminium wires into the codes as 

shown in the figure. 

% * 

2. Hold the two ends of the aluminium wires 
with the two hands. Bring the other two 
ends together. 

3. Ask your friend to hold a lighted candle at 


2. Why can you hold die wire B for a long 
time ? 

3. How was heat conducted along the wire A ? 

4. If you had held the wire A wrapped with 
paper or cloth, how would you have felt the 
heat ? 

Explanation 

1. Aluminium is a good conductor of heat 
Hence, it is heated quickly. Hence, you can 
not hold the wire A for long. 

2. Wire B has two parts connected through a 
cork. Cork is a very bad conductor of heat. 


% 




the junction of the two wires so that both 
the wires are equally heated. 

4. You will observe that it will be difficult for 
you to hold the single wire A for a long time. 
The wire A is heated much faster than the 
wire B with two smaller wires. 
Students to enquire 
1 . Why can not you hold the wire A for a long 
time ? 



Hence, the heat of the part heated by the 
candle can not reach quickly to the other 
part of the wire B held with the hand . 

3. The process of heat flow along the wire A 
is called conduction. 

4. Paper and cloth are bad conductors. Hence, 
if you hold a wire wrapped with paper or 
cloth, it will take much longer time for the 
heat to reach your hand. 

- 



Quest 49 




Which heats up first-2 


ii 









to 

Materials 

1. 15-20cm. long thin (2-3mm dia) wires of 
copper, aluminium and iron having the same 
length and diameter. 

2. Candle and matches. 


What to do ? 

1 . Tie one end of each wire together. 

2. Ask your friend to heat the junction of the 
wires by the lighted candle. 


2. Which of the parameters you have to keep 
constant ? 

3. For which difference in the property of the 
materials could you perform the 
experiment ? 

Explanation 

1 . Copper conducts heat the most and iron, the 
least. 

2 . In this experiment the length and diameters 




3. After half a minute or so touch the ends of 
the wires and try to observe which one is 
heated most You will observe that the copper 
wire will be heated the most and the iron 
wire, the least. 

Students to enquire 


mmm 


1 . Which metal conducts heat the fastest ? 


of the wires and the heat applied to the three 
wires were kept constant. 

3. The conductivity of different materials is 
different. The conductivity of a substance 
depends upon its molecular structure. The 
difference in the property of substances was 
utilised to conduct the experiment. 


Quest 50 




The cloth that does not 

burn 



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i 8 W 4 S« w 



ViwrOTwy^ 

•\ »_ m * f % % ^ to * * to 

w >. * ^ r >. * taj» \ i 



Materials 

1 . A 50 paise or 1 rupee coin 

2. A piece of old doth 

3. Candle and matches . 
What to do ? 


mm 


• • • c* ' ^ ■ * 7 ^. . . i 

r w . F »..»,»■ • to * • toj 

x<v:<‘X>:v>j 

to **k/to\Si^to J 


the coin for a long time, will the cloth bum ? 



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AVA 

aa 


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1 . Keep the cloth stretched and hold the candle 
on the cloth. Observe how easily it bums . 

2. Now, wrap the 

cloth tightly over -~prs. 

the coin and hold j. \ 

it with your left \ 

hand. Hold the — 
lighted candle on j/S 

the cloth for a tr (cs ' 

few seconds. ,S \\J 

The cloth is not 

burnt. / | / 

3. Remove the 

cloth and touch | 

the coin. You l j l 

will feel it hot. |i I 

Students to 1 


t-l 
Citi 


in 



i 


i 



enquire i® 

1 . Why is it easier ^ 

to bum the cloth 

without the coin ? . 

2. Why does not the cloth bum when the coin 
is there ? 

3. Why does the coin heat up ? 

4. If instead of a coin a piece of wood or 
plastic was taken, what would have hap- 
pened to the cloth ? 

5. If the candle is held on the cloth wrapped on 


mill 


Explanation 

1. The flame of the candle could raise the 

to 

* 

temperature of the cloth above its ignition 
point. Hence, the cloth burnt. 

2 . A metal is a good conductor of heat. Hence, 
when the lighted candle was held on die 

cloth wrapped 

& 

on a coin, the 
heat from the 
flame would 
gs conduct quickly 

/ to the coin. 

/ Owing to this the 

temperature of 


the cloth could 
not rise above 






the 


ignition 


r\ . .M point. Hence the 

\V<r cloth did not 

^ bum. 

" < 3. As a metal is a 

good conductor 
of heat, the coin 

got heated. 

4. Wood and plastic are bad conductors of 
heat. Hence, the temperature of cloth will 
rise above ignition point and the cloth will 
bum. 

5. If the candle is held on the cloth wrapped 
on the wire for a long time, the temperature 
of the cloth will rise above the ignition point 
and the cloth will bum. 


Quest 51 



Materials 

1 . A 4"x4 M piece of cloth 

2. A metal plate 
What to do ? 


+ • rnr S S *r ✓✓ rn m m m 0 m m 
► # * • •. 


1. Place one of your hands on the cloth and the 


Explanation 
1. A metal is a better conductor of heat 
than cloth. Normally, body temperature 
is more than the outside temperature. 


When you touch both the objects with 



other on the plate. You will observe that 
though both the materials are at the same 

room temperature, the metal plate feels 

* 

cooler. 


two hands the metal takes away heat from 
your body quicker than the cloth. Because 
of this your hand feels the metal cooler 
than the cloth. 



m 


Students to enquire 

1. Whydoyoufeeltheplatecoolerthanthecloth? 


Quest 52 







Materials 

1 . Four empty cold drink bottles 

2. Colouring material 
What to do ? 

1 . Fill the bottles A and B with cold water and 
the bottles C & D with hot water. 

2. Mix colouring material with the water of A 
and D. 

3. Close the mouths of the bottles B and C with 


your thumbs. Bring the bottles over the 
mouths of the bottles D and A respectively 
and take off your thumbs from the mouths 
of the bottles. 

4. After some time you will observe that the 
water in bottle B is getting coloured. But 
there is no change in the colour of the water 
in bottle C. 

Students to enquire 

1 . Why did the water in bottle B get coloured but 
why did not the water in the bottle C do so ? 





2. When will the water in bottle C get 
coloured ? 

3. If the water in all the bottles had been in the 
same temperature, then what changes would 
have happened to their colours ? 

4. Why did all the bottles come to the same 
temperature after some time ? 

Explanation 

1 . Density of hot water is less than cold water. 

So the cold water of bottle B comes 
down to bottle D and the hot water 
of bottle D moves up to bottle B by 
the process of convection. Thus the 
colourless water in bottle B gets 
coloured and the colour of water in 
bottle D becomes lighter. But the 
water in bottle C is hot and that in 
bottle A is cold. Hence the cold water 
in bottle A, being more dense, re- 
mains in the bottom and the warm 
water of bottle C tries to remain at 
the top. So the water in the bottles 
did not mix. 

2. When the temperatures of bottles A 
and C become equal owing to radiation, 
exchange of water takes place between the 
bottles through the process of diffusion and 
the water in bottle C gets coloured. 

3. When temperatures of all the bottles be- 
come the same, water of all the bottles be- 
come coloured through the process of dif- 
fusion. 

4. All the bottles will come to the same tem- 
perature by the process of radiation. 



- -***. 

Quest 53 



Materials 

1 . Crushed ice 

2. A large test 

■ 

3. Asmallstor 



2. The water along with the ice at the bottom 
of the test tube is cold. Hence, its density 
is more than the cold water. Hence, when 


lamp 


upper portion 
heated water 



rjf. r. 



What to do ? 

1. Fill one-third of the test tube with crushed 
ice. Keep the stone over the ice in the test 
tube to hold the ice. 

2. Take a little water in 

the test tube. 

• • 

3. Hold the test tube 

t * 

with the holder and 
heat the upper por- 
tion of the test tube 
until the water starts 


bottom. The 


convection process docs not work. So the 
ice always remains in contact with the cold 





boiling. 

4. You will observe that 

water boils at 100° C 
but the ice does not 
melt 

Students to enquire 

1 . What is the purpose of keeping the stone ? 

2. Why does not the ice melt when the water 

boils ? 

3. If the stone were not there, could the ex- 
periment be conducted ? 

4. Was the water conducting the heat well ? 

5. Did the convection process work in 

water ? 

Explanation 

1 . Ice could not float up because of the stone 

over it. 




water. Hence, the ice does not melt. 

3. The experiment could not be performed if 
the stone were not there. Without the stone 
the ice would float up and it would melt 
owing to the convection process. 

4. Water is a bad conductor of heat. Hence, 

when the upper portion of the test tube was 

♦ 

heated, the water did not conduct the heat 
to the ice well. 

4. The convection process was not working as 


always try 


bottom 



Quest 54 



Materials 


1. Two glass tumblers 



thermometers 



3. A piece of white paper and a piece of used 


carbon 


spot light 


What 



1. Cover one glass with the piece of white 
paper and the other glass with the piece of 
carbon DaDer. 


records the higher temperature. 

Students to enquire 

1 . Why does the glass covered with the carbon 
paper get heated more ? 

2. Normally, which colours of light are more 
hot and cold ? 

Explanation 

1 . A black body absorbs more radiant energy 
than a white body. A white body reflects all 
colours of light but a black body absorbs 





Take the same amount of water in the two 

glasses and introduce the two thermometers 
into the glasses. 

3. Turn the spotlight on and keep the glasses 
equidistant from the light so that both may 

receive the same amount of light from the 
lamp. 

4. After 5-6 minutes record the temperatures 
measured by the two thermometers. You 
will observe that the thermometer in the 
glass that is covered with carbon paper 


all colours. The glass covered with the piece 
of black carbon paper absorbs more radiant 
energy from the lamp but the other glass 
covered with the piece of white paper re- 
flects away all colours. Hence, the water in 
the glass covered with the carbon paper 
gets heated more. 

2. The warmer colours in descending order 
are : red, orange, brown and yellow. The 
sequence of cooler colours is : green, blue, 
purple and violet. 



Quest 55 





Materials 

1 . Two similar thermometers 

2. A piece of cloth 

3. A glass and some water 

What to do ? 

1. Record the temperatures measured by the 
two thermometers. 

% 

2. Cover the bulb of one of the thermometers 
with a piece of wet cloth. Take water in the 
glass and dip this thermom- 
eter in water. 

Note the temperature as mea- 
sured by the thermometer. 

3. Note the relative humidity 
from the weather reports in 

the newspaper. 

Students to enquire MM 

1 . Is there any difference between 
the temperatures recorded by 
the dry bulb thermometer and 
that with the wet bulb ? If so, 


why 


? 


2. If the relative humidity is high, 
what difference between the 
temperatures measured by the 
two thermometers will be 

noticed ? 

3. If the difference between the two tempera- 
tures is much, what inference can you draw 
about the relative humidity in the air ? 


For evaporation the water draws heat from 
its environment including the bulb of the 
thermometer. For this the thermometer gets 
cooled and there is a difference between the 
dry and wet bulb thermometers.The drier 
the air, the quicker is the evaporation and 
the lower the temperature reading in the wet 
bulb thermometer. 

2. If the relative humidity in the air is high, 




When the bulb of a thermometer is wet, 
evaporation takes place from the wet cloth. 


then there is practically no difference in 
temperature between the dry & wet bulb 
thermometers. 

3. If the difference between the dry and wet 
bulb temperatures is high, then it proves 
that the relative humidity is low. 



Quest 56 






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Heat and temperature 


Materials 

1. Two elas 




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5SSSSSSSS 




tumblers 


2. A heavy piece of iron 

3. A big nail 


4. 


thermometers 


5. Two toys 

6. Two heaters 

7. A beaker. 


water in the glasses. 

3. When the water in the beaker starts boiling 
replace the piece of iron and the nail from 
the beaker to the two glasses with the aid 


Keep the thermometers 


the water. 
Record the 


till the rise in temperature stops. 



What to do ? 

1 . Put the piece of iron and the nail in the water 
in a beaker and boil the water. 

2. Take the same amount of water (100 c.c.) 
in the two glasses. Measure temperatures 
by placing the two thermometers into the 


Students to enquire 

1 . What were the temperatures of water in the 
two glasses before the introduction of the 
piece of hot iron and the nail ? 

2. When the water starts boiling what is its 
temperature ? 



Quest 57 




3. After the hot iron objects are transferred to 
the glasses, water in which glass becomes 
hotter ? 

4. Why did the temperatures of water in the 
glasses rise ? 

5 . Out of the two iron pieces which one has 


more thermal capacity ? 


water in the glasses the one with the bigger 
object rises to a greater temperature com- 
pared to the other one. 


4. The 


the conduction of heat from the hot object 


to the water. 


5. The bigger iron object has greater thermal 



What are the factors on which thermal 
capacity depends ? 


1. Before the hot metal objects were placed, 
the temperatures of water in both the glasses 
were the same, i.e., the normal room tem- 
perature . 

2. The temperature of boiling water is 100°C. 

3. When the hot objects are transferred to the 


capacity than the smaller one because of its 
greater mass. So heat in the bigger object 
is more than the smaller one. Hence, the 
water in the glass in which the bigger object 
is placed rises to a greater temperature. 

6. The thermal capacity of an object depends 
upon its mass and the material of which the 

object is made. 



Quest 58 



wm 



jm 








Materials 

1. A 100 ml conical flask with a rubber stop- 
per and a tube passing through it 

2. A heater or a spirit-lamp 

3. Ink and crushed ice 

What to do ? 

1 . Fill the flask with coloured water. Gose the 
mouth of the flask tightly with the stopper. 

2. Heat the water in the flask. You will observe 
that the coloured water is rising in the tube. 
Be careful that air cannot enter the flask. 

3 . Now, keep the flask in crushed ice. You will 
observe that the coloured water in the tube 
is going down. 

Students to enquire 

1 . Why do you have to stop entry of air inside 
the flask ? 

2. Why is there a change in the height of water 
in the tube due to heating or cooling ? 

3. Can you compare it with a real 
thermometer ? 

4. How can you calibrate this improvised ther- 
mometer ? 

Explanation 

1 . The flask has to be air tight so that when 
the water expands as a result of heating it 
cannot escape from anywhere except through 
the tube. 



2. Most of the materials -solids, liquids or gases 
expand on heating and contract on cooling. 
As water cannot escape, it rises in the tube on 
heating and comes down on cooling. 

3. It is an analogue of a thermometer. 


4. To calibrate the thermometer keep the flask 
on crushed ice. You will observe that the 
water will start coming down and will stop 
at a certain height. Tie a rubber band to 
show the height. That will mark 0°C. Take 
out the flask from ice and after some time 
heat the flask for the water to boil. Water 



will rise in the tube and stop at a particular 
height. Tie a rubber band to show that height. 
That marics 100°C.Now you may divide 0° 
to 100° at equal intervals. You have made 
your own thermometer. 



Quest 59 





Materials 

1. An alcohol thermometer 

2. Ice cubes 

3. A heater 

4. A beaker 


What to do ? 


msm 
mm 




1. Take a few ice cubes in a beaker and 
introduce the thermometer into the ice cubes 
(fig.A). Observe what happens to the alco- 
hol in the thermometer. Keep measuring the 
temperature at intervals of one minute till 
all the ice melts into water. Measure the 


4. Measure the temperature of the steam that 
comes out of the boiling water. 



: msm 


Students to enquire 

1 . When the thermometer is kept in the ice for 
some time, what is the temperature of ice? 

2. What is the temperature of ice till it melts 
completely ? 

3. What is the temperature of boiling water ? 

4. What is the temperature of the melted water 
after it has been kept for some time ? 

5. What is the temperature of the steam com- 
ing out of the boiling water ? 



temperature of the melted water after 10 or 
15 minutes. 

2. Now place the beaker with water on the 
heater (fig. B), insert the thermometer in 
water and heat it. Do not insert the ther- 
mometer in boiling water. 

3. Observe what happens to the alcohol in the 
thermometer. Observe the temperature when 
the water starts boiling. Keep the water 
boiling for half a minute and observe what 
happens to the temperature. 



Explanation 

1 . When the thermometer is kept in the ice for 
some time it will show 0°C temperature. 

2. As long as all the ice is not melted, the 
thermometer will show 0°C temperature. 

3. The temperature of boiling water is 100°C. 

4. When the melted water is kept for some 
time, the temperature gradually increases. 

5. The temperature of steam coming out of 
boiling water is 100°C. 



Quest 60 













rfU lit 


mmm&m 



y.v. 






*as 


Melting point 


«AV 5 


■ 


wlllil 




lililM 





•0.0.0 


A 


Jlllip 


pp* 1 


iw 


Materials 

1 . A big block of ice 

2. A one meter long wire 

3. Two bricks or heavy objects 

What to do ? IIHS««^« 
' 1 . Keep the block of ice on some object in 
such a way that its edges project outside. 

2. Tie two heavy weights at the two ends of 
the wire and hang it in such a way that the 
wire passes through the centre of the ice 
block. 


the pressure of wire on the ice ? 

If the ice block is small, will the wire cut 
through the ice quicker ? 

If the weights connected to the wire is in- 
creased, will the wire cut through the ice 
quicker ? If so, why ? 









3 . After some time you will observe that the 
wire has cut through the ice. But as the wire 
cuts through, the ice above the wire joins 
together. 

4. When the wire cuts completely through the 
ice, it again joins together into one block. 






Students to enquire 18 B— 1^^^ 

1. Why docs the wire cut through the ice ? 

2. What happened to the melted water above 
the ice ? 

3 . Which property of the ice is changed due to 


Explanation mmmmMmmmmmmM 

1 . The melting point of a solid is lowered when 
the pressure on it is increased. The wire 
exerts pressure on the ice. Hence the melting 
point of ice, where the wire exerts pressure 

on the ice, is de- 
creased. For this the 
ice below the wire 
— ] melts and the wire 

/ cuts through the ice. 

2. After the ice melts 
/ because of the low- 

ering of the melting 
point and the wire 
cuts through the 
s* melted water which 

is at 0°C the water 

freezes up again. 

3. If pressure is increased on a solid, its melt- 
ing point decreases. 

4. If the block is small and the weight on the 
wire remains the same, then the wire will cut 
through quickly. For, in such a case pres- 
sure at a particular point will increase. 

5. If the weight connected to the wire is in- 
creased, then the wire will cut through the 
ice quickly because pressure at a particular 
point will increase. 


Quest 61 








Materials 

1. A small (100 c.c.) beaker 

2. A thermometer and a spoon 

3. A heater or a spirit-lamp 

What to do ? 

1 . Heat about 2cc of water in a beaker. Mea- 
sure the temperature of the water every half 
a minute and keep a note of it. 

2. When the water starts boiling, mix about 2 
gms. of salt with the water. 

3 . Continue heating the water and 
stir the water with the spoon. 

Note the temperature of water 
every half a minute till the 
water starts boiling. 

4. Repeat the same experiment 
with 4 gms. and 6 gms. of salt. 

Students to enquire 

1. What is the boiling point of 
water at normal atmospheric 
pressure ? 

2. After salt is mixed, what hap- 
pens to the temperature of the 
solution ? 

3. What is the boiling point of 
the solution ? 

4. Why does the boiling point increase ? 

5. What happens when you repeat the experi- 
ment with 4 gms. or 6 gms. of salt ? 

Explanation 

1. At ordinary pressure the boiling point of 
water is 100°C. 

2. When salt is mixed with the water while it 




is boiling, the boiling stops and the tem- 
perature of water falls. Salt takes some heat 
from the water to melt. Because of this the 
temperature falls and the boiling stops. 

3. The boiling point of the solution is increased. 

4. Salt gets ionised in water. Water molecules 
keep together owing to cohesion. Water 
molecules attach also to sodium chloride 
ions. Owing to this it becomes difficult to 



conduct heat and the boiling stops. This 
causes the increase of the boiling point. 

5 . The more the salt, the more is the increase 
of the boiling point of the solution. But if 
the temperature of the steam coming out is 
measured, it will be 100°C. 


Quest 62 







Materials 

1. A glass tumbler and a spoon 

2. A thermometer 

3. Ice cubes and water 
What to do ? 

1. Take a glass of water and drop a few ice 

cubes in it Keep a thermometer in the water 
and note the temperature at regular inter- 
vals. 




humidity in the air is low ? 

4. Where do we find a similar event in our 
daily life ? 

Explanation 

1 . Dew comes from the air. Ice cools the glass. 
So, the water vapour in the atmosphere gets 
condensed into water droplets when it comes 
in contact with the cool glass. 

2. If the dew point is measured in different 



2. Observe the outside of the glass carefully. 

3. Stir the water with the spoon and observe 
at what temperature the dew appears out- 
side the glass. 



Students to enquire 

1. From where does the dew come on the 
outside of the glass ? 

2. At what temperature is the dew formed ? 

3. Will the dew point be higher or lower if the 


seasons at different periods of the day or the 
year, it will be observed that the dew point 
will be different. 

3 . The drier the air, the lower is the dew point. 

4 . In winter we observe that dew forms on the 
grass early in the morning. In the night the 
earth cools and when the water vapour in 
the air comes in contact with the cool earth, 
it condenses into water droplets. 





Quest 63 








mmmm 


Materials 

1. A cup 

2. A thermometer (-10°C to 50°C) 

3. Salt 

4. Ice 

What to do ? 

1. Fill the cup with ice 
cubes and insert a ther- 
mometer in the ice. 

Crushed ice will be bet- 
ter. 

2. You will observe that 
after some time the ther- 
mometer will show 0°C 
temperature. The ther- 
mometer will not go 
down further, no matter 
how long you keep the 
thermometer in the ice. 

3. Now, mix salt with the 
ice and stir the mixture. Notice the ther- 
mometer reading. You will observe that the 
terriperature will go down to about -10°C. 
Some of the ice will melt quickly. 

Students to enquire 






Explanation 

1. Normally, ice melts at 0°C. 

2. The melting or freezing point of ice and salt 
solution is always lower than pure water. 
So, when salt is mixed, the ice melts at 0°C 




mmm 





1. Normally, at what temperature does ice 
melt ? 

2. Why does ice melt when you mix salt with 
it ? 

3. When ice melts on addition of salt, is heat 
added or extracted ? 

4. Is heat added or extracted while the ex- 
periment is conducted ? 


as the freezing point gets lowered. 

3 . Ice needs heat to melt. But as no heat has 
been aided to the ice from outside, it ex- 
tracts heat from its environment to melt. A 
similar event occurs when water is kept in 
an earthen pitcher. Water oozes through the 


evaporates 


pot 


earthen pitcher 


is cool during summer. 


4 . In this experiment no heat has been supplied 
from outside. Heat has been extracted from 
the ice itself. 


Quest 64 




Materials 

1 . Ether or alcohol 

2. A* straw 

3. A thin steel glass 

4. A thin wooden board 

1 . Wet the wooden board with water. 

2. Take a little ether or alcohol in the glass and 

keeD it on the wooden board. 



w. 


Students to enquire 

1. Why does the glass get cooled when you 
blow air through the alcohol or ether ? 

2. Why does the glass get stuck to the board ? 

3. Why does the experiment work better with 
coconut oil ? 




1 . When the air is blown through the alcohol 
or ether, it gets evaporated quickly. For 



3. Now insert a straw in the liquid and keep 
blowing air through it. You will observe 
that bubbles will come up through the ether. 

4. After blowing for a while, you will observe 
that the glass has become very cool and it 
has stuck to the board. If you raise the glass 
the board also will come up with it. 

5. If you wet the board with coconut oil.then 
the experiment will work better. 


evaporation latent heat is required. The latent 
heat is taken away from the ether or alcohol 
and glass. Because of this the liquid and the 
glass get cooled. 

2 . When the glass becomes too cold, the water 
in the board below the glass gets cooled and 
forms ice. For this the glass gets stuck to 
the board. 


Be careful : Ether or alcohol is highly inflammable. 



Quest 65 








Materials 

1. A table fan 

2. Two alcohol thermometers 
What to do ? 





1 . Note the temperatures recorded by the ther- 


mometers. 

2. Keep the thermometer A in the wind in front 
of the fan. Keep the thermometer B at the 
back of the fan. 



3. Keep the fan blowing for some time and 
then mark the temperatures recorded by both 
the thermometers. You will observe that 
both the thermometers show the same read- 


ing. 

4. Now wrap the bulb of the thermometer A 
with a wet cloth and switch on the fan. If 
the temperature is measured after a few 
minutes you will observe that the tempera- 
ture recorded by the thermometer A is less 
than that recorded by thermometer B. 




Students to enquire 

1 . When the fan is turned on before the bulb 
of the thermometer A has been covered with 
the wet cloth, why is there no difference 
between the temperatures recorded by the 
two thermometers ? 

2. When the bulb of the thermometer A is 
wrapped with the wet cloth, why does it 
record a lower temperature ? 

3. Why do we feel cool when the air blows ? 



sills 




Explanation | 

1 . Apparently, it might seem that when air 

blows, the thermometer A should reach a 
lower temperature. But if the temperature is 


thermometer 


the thermometer A will 


temperature. The temperature in thermom- 
eter A will rise because of friction between 
the thermometer bulb and the blowing air. 

2. But when the bulb is wrapped with the wet 

cloth, water evaporates from the cloth. For 
evaporationheatis taken away from thebulb. 
When air blows from the fan, the saturated 
ai r round the bulb is driven away . Hence a 
lower temperature is shown by the thermom- 
eter A. 

3. When the air blows, sweat evaporates more 
quickly. Hence we feel cool. During the 
summer when the air is humid, the air round 


our body remains saturated with water 
vapour. Then the evaporation of sweat from 
the body stops. Hence we feel uncomfort- 
able. But. if the air blows, the saturated 
air is driven away. More evaporation of 
sweat takes place and we feel comfortable. 



Quest 66 














mr 


Materials 

1. An ice cube 

2. Salt 

3 . A long thread 
What to do ? 

1 . Keep the ice on the table and place a thread 
along the centre line of the ice cube. 

2. Sprinkle some salt on the thread. 

3. After a little while try to raise the thread 
carefully. You will observe that the ice cube 






will come up along with the thread. 

4. You will observe that the ice has melted at 
the place where the salt is sprinkled. But the 
melted water will solidify into ice again 
after a while. 





Students to enquire 

1 . At what temperature does the ice melt ? 

2. How could you lift the ice cube ? 

3. Where in our daily life do we use the prop- 
erty of lowering the melting point of ice with 
the help of salt ? 

4. In cold countries when the road is covered 
with snow, salt is sprinkled on the road, why? 



Explanation 

1 . Ice melts at 0°C. 

2 . The melting or freezing point of saline water 
is less than pure water. Hence, when salt is 
sprinkled, the ice around the thread melts.The 
temperature of melted water is less than 0°C 

9 

When a considerable quantity of ice melts, 
the concentration of salinity of water re- 
duces. For this the freezing point of the 
solution comes near to 0°C. But as the tem- 
perature of the water is then a few degrees 
below 0°C it freezes again and the thread 
gets stuck to the ice. 

3. When ice cream is made at home, a tem- 
perature lower than 0°C is achieved by 
mixing salt with the crushed ice. Owing to 
this the liquid mixture that makes the ice 


4. 


cream 


formed 


roads during winter time it becomes diffi- 
cult to drive a car. The car skids while 
moving. When the car moves on ice it 
exerts pressure on the ice. We know that the 
freezing point of water goes down with 
increased pressure. Because of this, the ice 
below the tyres melts, the road becomes 
slippery and the car skids. To avoid skid- 
ding salt is sprinkled on the ice. Salt lowers 
the temperature of ice below 20°C. Hence, 
when a car moves on the ice, though the 
freezing point of water goes below 0°C the 
ice does not melt as its temperature is al- 
ready much below 0°C because of the sprin- 
kling of salt. 


Quest 67 





Materials 

1. Matches 

2. Ice cubes 

3. A small beaker(100cc) 

4. A spirit lamp or a heater 

5. A candle 

What to do ? 

1 . Light the candle. You will observe that wax 
will start melting. After some time you will 
observe that gas is com- 
ing out of the molten 
wax. 

2. Blow off the candle. 

You will observe that 
the gas is coming out. 

Bring a lighted 
matchstick near the 
gas. You will observe 
that the candle will 
catch the flame. 

3. Take a small piece of 
ice in a beaker and heat 
it. You will observe that 
the ice will melt into 
water and after some 
time water will convert 
into steam. 

Students to enquire 

1 . What do you understand by the melting and 
gasification of wax by heat ? 

2 . Why does the candle burst into flame when 
you bring a lighted matchstick near it when 
it has just been put out ? 


What 


formation of steam 


to heat ? 

Explanation 



formation 


gas one can understand that heat changes 
the state of matter. 

2. After a candle is put out, wax gas comes 
out for some time .When a lighted matchstick 



ignited 


flame 


3. The cl 
steam 


matter. It also shows that matter has three 
states - solid, liquid and gaseous. 



Quest 68 











Materials 
1. A 3 m. long iron wire 
2 A 500 gm. weight 

3. A scale 

4. Matches 

5. Six candles 

6. Two chairs 

7. A paper arrow 

8 . A clip or a little cellotape 





* 

2. Will there be any change in the amount of 
lowering of the arrow if the numbers of 
candles are changed ? 

3. Will the amount of lowering depend upon 
the material of the wire ? 

4. Give some examples of expansion by heat 
from your daily experience. 

Explanation 

1 . When you heat the wire, it expands in length. 








Jill 


What to do ? 

1 . Tie the wire to the chairs and keep it taut. 
Ask two of your friends to sit on the chairs. 

2. Hang the weight with a string from the 
middle of the wire. Attach the paper arrow 
to the string or the weight with the clip or 
the cellotape respectively so that it may 
show height on the scale. 

3. Now, ask your friends to heat the wire at 
different places with the lighted candles. 
You will observe that the arrow will gradu- 
ally go down. 

Students to enquire 

1. Why will the arrow go down ? 





Hence the arrow will go down. 

2. The wire will extend more if the number 

/ 

of candles are increased. The arrow will go 
down. 

3. Different materials expand differently with 
the same amount of heat. Copper will expand 
more. 

4. To prevent the railway lines from buck- 

• ^ 

ling, a small gap is left between two suc- 
cessive rails to allow for expansion of the 
lines when heated. The same precaution is 
taken when placing steel bands round cart- 
wheels. 



Quest 69 




Materials 

1. A 50 c.c. pipette 

2. A glass tumbler 

3. A heater 
What to do ? 




!»■ 

mm & * 




1. Heat some water and take the hot water in 

the glass. 



Students to enquire 

1. Why does the watercome out like a spray ? 

2 . Why is it that the water does not come out like 
a spray when you invert the pipette slowly ? 

Explanation 
1 . When the pipette is suddenly inverted and 

hot water starts descending, it heats up the 




2. Draw by mouth the hot water in the pipette 
(half of it). 

3. Then close the open end with your thumb 
and quickly draw out the pipette from the 
hot water and invert it. 

4. You will observe that hot water comes out 
of the nozzle end like a spray. 

5. Invert the pipette slowly and observe what 

happens. 


This 


This 


So 


water comes out like a spray. 
2. When you invert the pipette 


air does not get trapped. Hence a higher 
pressure is not created to force the water 

out. 




Quest 70 









Materials 

1 . An empty shoe-box 

2. A candle, a match-box and some incense 
sticks (agarbattis) 

3. A transparent plastic sheet 

4 . Two hard cardboard cylinders ( 1 5 cm. high) 

5. Some cellotape 






What to do ? 

1 . Cut open one long side of the shoe-box and 
cover it with the plastic sheet so that the 
inside may be visible. Fix the two cylinders 
on the top of the box as shown in the figure. 

2. Place the candle first below the chimney A 
and light the candle. If there is any opening 
cover it with cellotape. 

3 . Light two incense sticks and hold them over 
the chimney B. Do not hold the sticks just 
above the chimney. Blow the incense smoke 
near the chimney B. Observe how the smoke 
travels. 


Students to enquire 

1 . Why does the smoke come down the chim- 
ney B ? 

2. How can you make the air circulate between 
the two chimneys without the lighted candle? 

3 . Where on earth do we find an example of 
this experiment ? 

Explanation 

1. The air around the 
lighted candle gets 
warm, becomes light 
and rises up the chim- 
ney A. Owing to this a 
partial vacuum is cre- 
ated inside the box. 
This causes the incense 
smoke to enter through 
the chimney B into the 
box to fill up the 
vacuum. A draught is created inside from 
chimney B to chimney A and the smoke 
goes up the chimney A. 

2. If we hold an ice cube over the chimney B, 
then the air round it becomes heavy and will 
go down the chimney B and a wind current 
will be created. 

3. In the daytime the land gets heated and the light 
air goes up creating a partial vacuum. Cool 
air from the sea rushes towards the land cre- 
ating an air current. In the night the land be- 
comes cool quicker than the sea. Then the cool 
air from the land blows towards the sea. 


Quest 71 











Materials 
1. A powerful torch 
2 : A daik room 


What 




1 . Focus the torchlight straight on the wall and 
notice the brightness of the spot of light. 

2. Now throw the light on the same spot 
obliquely. You will 
observe that the light 
gets spread over a 
wider area and the 
brightness per unit area 
has decreased. 

Students to enquire 

1. Why does the bright- 
ness differ when the 
light falls on the wall 
obliquely ? 

2. What relationship is 
there between this 
experiment and the 
change of season ? 

Explanation 

1. The amount of light 
energy coming out of 
the torch remains the 
same whether it falls straignt or ootiqueiy 
on the wall. But when the light falls 
slantingly it is distributed over a wider area. 
Hence the amount of light per unit area is 
less. Because of this the brightness is de- 
creased when light falls slantingly. 


2. When the earth resolves round the sun, the 
same phenomenon occurs . The axis of earth 
is at an angle of 66 %° with the ecliptic 
plane. From 22nd March to 23rd September 
the sunlight falls at right angles on different 
latitudes on earth upto 237 2 °N. Hence the 
northern hemisphere has summer during this 




period. But from 23rd September to 22nd 
March the sunlight falls at right angles on 
different latitudes in the southern hemisphere. 
During this period the southern hemisphere 
has summer and in the northern hemisphere 
where the sunlight falls obliquely is winter. 



Quest 72 


v/?^y^A<<-VA</^M\w y/Mv&uxt. 


SSSi 



/ ■ PL* ■ .P • » • f 7 'a S a v a • * » r. A*i r'^fcV’F* * 

r • • * • • • * »-•-*•' • A A A AT M ■ • ■ 

AV.' * * •*••••». ^ J a wx w a r^H 


1 


■ •>»:*:.*: ;• : v >; . •; 

< tu 5 tx< $ >cj g rs 






Magnetic and 
Non-magnetic substances 


v*v*v 






Materials 

1 . A bar magnet 

2. Different materials; a nail; a pin, a piece of 
rubber, a balloon, a piece of chalk, a piece 
of copper wire, a piece of aluminium foil, 
a matchstick, a marble, a 10 paise coin and 


a piece of paper 


not attracted by the magnet 
Students to enquire 




1 . What are the names of the objects that are 
attracted by the magnet ? 

2. What are the names of the objects that are 
not attracted by the magnet ? 


3. Prepare a table of objects based on their 



What 


w 

1 . Spread the objects on the table. Touch each 
object with the magnet one by one. 

2. Keep the objects which are attracted by the 


magnetic properties. 
Explanation 





magnet 


are 


1 . The objects which are attracted by the mag- 
net are called magnetic substances. 

2. The objects which are not attracted by the 
magnet are called non-magnetic substances. 


Quest 73 







Materials 

1 . A bar magnet 

2. Iron filings 

What to do ? 

1 . Take some iron filings on a piece of paper 
and spread it evenly. 

2. Bring the magnet over the filings but b e 
careful not to touch them. 

Observe where in the 
magnet the filings are 
attracted most. 

3. The experiment works 
well if the bar magnet is 
long (4"-6") and is weak. 

Students to enquire 

1. Where do most of the 


middle portion practically no ir 
are attracted. If the bar magnet 
nd lone, the effect can be seen mu 


better. 


3. The ends where the attraction is the maxi- 
mum are called the poles of a magnet. 


The central portion of the magnet where no 



filings accumulate on the 


magnet 


2 


portion 


cally no iron filings are 
attracted ? 

3. What are the ends where 
most of the iron filings 
are collected called ? 

4. What is the name of that 
portion of a magnet where 
no iron filings are at- 
tracted ? 

5. Which property of a magnet is understood 
by this experiment ? 



Explanation ■■■■MHI 

1. Most of the filings are attracted at the ends 

of a magnet. 


neutral 


zone. 


5. The attractive property of a magnet is 
undetstood by this experiment. 



Quest 74 





Materials 

1 . A long bar magnet 

2. A bent copper piece tied to a string 
What to do ? 


■MS: 


fill 


1 . Place the bar magnet on the bent copper 
piece and allow it to hang horizontally by 
holding the string. 


will see that the magnet again turns and 
shows the north-south direction. 



Students to enquire S 
1 . Why does the magnet come to show the 
north-south direction every time ? 





H* 



Explanation 
1 . Earth behaves as a big magnet with its poles 



2. Observe that the magnet swings and then it 
shows the north-south direction when it 
comes to rest. 

3. Swing the magnet slightly and observe that 
it again shows the north-south direction. 

4. Now, reverse the magnet and hang it. You 


situated near the geographical north and 
south. The magnetic field of the earth aligns 
the hung magnet in the north-south direc- 
tion. The end which points towards the north 
is designated the north pole and the other 
end, the south pole. 


Quest 75 






Materials 


1 . A bar magnet 

2. A magnetic needle 
What to do ? 




1. Keep the magnetic needle on the table. When 
the needle comes to rest observe the direc- 
tion the needle shows. 


2. Why does the needle point to the same 
direction ? 

Explanation 




V •’ SS.’S . \ ■-V.r.V/ 


1 . The needle always points towards the north- 
south direction. 

2. A needle is a very feeble magnet which can 
move freely on a pivot. The earth behaves 



2. Keep the bar magnet about 15 cm. above 
the needle and rotate it so that the needle 
starts rotating. 

3. Stop the rotation of the needle and observe 
the direction the needle shows. 



Students to enquire 

1 . To which direction does the needle point ? 


like a huge bar magnet. The earth’s mag- 
netic field aligns the magnetic needle in the 
north-south direction. The end of the needle 
which points towards the geographical North 
is called the North pole and the other end 
which points southwards is called the South 
pole. 




Quest 76 




Magnetic attraction & 

repulsion 




Materials 
1. Two magnets 

What to do ? 

1 . Normally, a bar magnet has in one end a 
white spot which shows that the end is north. 





4. Bring the north end of one magnet near the 
south end of the other. Both magnets will 
be attracted to each other. 

Students to enquire 
1 . Which is the surer test of magnetization — 




Then the other end will be south. 

Now bring the north end of one magnet near 
the north end of the other magnet It will be 
seen that the magnets are repelled. 
Similarly repulsion occurs if south ends are 
brought together. 



attraction or repulsion ? 

Explanation 
1 . The surer test of magnetism is repulsion, 
not attraction. A magnet will attract a mag- 
netic substance at both ends. A magnetic 
substance cannot be repelled by a magnet. 



Quest 77 



Materials 


1 . A strong bar magnet 

2. A long iron nail 

3. Small nails 


What to do ? 






mm 


1. Place small nails in a heap on a table. 

2. Touch a bar magnet on the long iron nail 
and then bring the pointed end of the nail 
near the heap of 
small nails. 

3. The small nails will 
cling to the long nail 
in a chain. 

4. Remove the magnet 
from the long nail. 

The small nails will 
immediately fall. 

Students to 
enquire 

1. Why does the long 

nail attract 

* 

smaller nail when 
the magnet touches 
the long nail ? 

2. Will a copper nail 
or wooden nail at- 
tract small nails ? 

3. What is the process 

zation called ? 


The long nail will attract the small nails as 
long as the bar magnet touches the long nail. 

The nail becomes a temporary magnet. 

■ 

2. A copper or wooden nail will not become 
a magnet when a magnet touches it. Hence 
it will not attract the small nails. 

3. The process of temporary magnetization is 

9 

called induction. The induced magnetism is 





wmmmmmmmrnm 




Explanation 

1 . When the magnet touches the long iron nail, 
it becomes a magnet. Hence it attracts the 
small nails or any other magnetic material. 


The tiny molecular magnets in 

A 

[ align themselves systematically 
he magnet touches the nail, but 
lar magnets become haphazard 


when the magnet is removed. 


. 


Quest 78 







1. Small iron nails 5-10 pieces 

2. A bar magnet 

w 

3 . An exercise book 

4. A glass of water 

9 

1. Place the nails over the exercise book. 

2. Place the bar magnet below the exercise 




magnet and hold it outside near the bottom 
of the glass and rotate it round the glass. 
What haDDens ? 


Students to enquire 

1. Does the magnet affect the iron nails through 

the exercise book ? 

2. Docs the magnet affect the iron nails through 
the water ? 


fl, 




3. 


book, observe that the magnet affects the 
iron nails when the magnet is moved below 
the exercise book. 

Take 3 / 4 lh glass of water and drop a few 
iron nails in the water. Now take the bar 


Explanation 


llllill 




1 . Yes, the iron nails are affected by the magnet 
through the exercise book. 

2 . Yes, the magnet affects the iron nails through 
the water. 


Quest 79 




Materials 

1 . A bar magnet 

2. An iron bar resembling a bar magnet 



What to 

1. You have to identify which one is the 
magnet and which 
one is an iron bar. 

2. Touch the end of 
one bar at points 
A, B, C of the 
other bar. 

3. Observe what hap- 
pens. 

4. Now change the 
bars and do the 

9 

same operation 
and observe the 
nature of attraction 
at the ends and in 
the middle. 

Students to 

enquire 

1. Why will one bar 
attract the other 
bar at all points ? 

What does that 
prove ? 

2. When you reverse 

the bars you will 
observe there is no attraction in the middle, 



why 





Explanation 

1 . If the bar magnet is touched at points A, 


B and C of the other bar, then it will 


points 


magnet 


one is an iron bar. 


0 

2. If the iron bar is touched at points A, B 



and C of the magnet, then there will be 
attraction at A and C, but no attraction at 
B. The centre of a magnet is a neutral 
point. 



Quest 80 





Materials 

1 . A used safety razor blade 

P 

2. A strong bar magnet 

3. A magnetic needle 
What to do ? 

1 . Place the blade on the table and hold it with 
a finger. 

2. Hold the bar magnet vertically and press it 
on the blade at one end. 




Students to enquire 

1 . Why does the blade become a magnet ? 

2. If you rub the north end of the magnet, what 
will be the polarity at A and B ? 

3. If the end of a blade after magnetisation 
repels die north end of a magnetic needle, 
what polarity is formed at that part of the 
blade ? 

4. What polarity is formed in the other end ? 



3. Then draw the magnet from end A to end 
B and then raise the magnet and draw it 
again from A to B. 

4. Do it four or five times. The blade will 
become a magnet. 

5. Bring one end of the blade near the north 
end of a magnetic needle. Observe what 
happpens. 

6. Now bring the other end of the blade near 
the north end of the magnetic needle. Observe 
what happens. 


1. When the blade is rubbed with the magnet 
each tiny molecular magnet in the blade 
gets aligned systematically and becomes a 
magnet. 

2. The end A becomes the north and the end 
B becomes the south. 

3. If an end of a blade after magnetization 
repels the north end of a magnetic needle, 
then that end is the north pole. 

4. The other end of the blade is the south pole. 



Quest 81 



Molecular magnetism 


d 


Materials iiiiMiii 

1. A test tube 

2. Iron filings 

3. A bar magnet 

4. A magnetic needle 





What to do 

1. Fill the tes 


m 


















fourths 


filings. Close the mouth of the test tube 
with paper. 

2. Spread the iron filings evenly in the test tube 

♦ 

and keep it horizontal. 

3. Take one end of the test tube at right angle 
to the needle towards the north and the south 
end of the needle and observe I 


same 


pole 


4. 


north 


magnet over one end of the test 
tube and take the magnet along 
the test tube to the other end. 

Repeat the operation three or 

four times. 

5. Now, take the end of the test 

tube, from where the rubbing 

was started, towards the north pole of the 
magnetic needle. You will observe that the 
needle will be repelled. 

6. Now, bring the other end of the test tube 
towards the north pole. You will observe 
that the needle will be attracted. It proves 
that the test tube behaves like a bar magnet. 

7. Now, shake the test tube and repeat the 
operation. You will observe that the needle 
would not be deflected .This proves that the 
test tube does not behave like a bar magnet. 


Students to enquire MHMMi 

1 . Before magnetization why did both the ends 

of the test tube get attracted by the magnetic 

♦ 

needle ? 

2. How was the test tube converted into a 
magnet ? 

3. How does this experiment explain the mo- 
lecular theory of magnetism ? 

Explanation 


magnetic 


The 


the ends are attracted by the magnetic needle. 
When the test tube is rubbed with a magnet. 




magnetised 


arrange 


behaves like a bar magnet. 


3. 


All the molecules in a magnetic substance 
are small magnets. Normally, the molecular 
magnets are arranged in domains in such a 
way that the substance does not show any 
magnetic property. But when the magnetic 
substance is magnetized, the molecular 
magnets align themselves in such a way that 
the substance shows magnetic property. 



Quest 82 













Materials 

1 . A bar magnet 

2. A plastic sheet 

3. A thin iron sheet 

4. A few small nails 
What to do ? 

1 . Hold the magnet vertically and place the 

plastic sheet below the magnet. Now, bring 
the magnet with the plastic sheet near the 
nails. You will observe that the nails are 
attracted by the magnet and they stick to the 



2. Why do the nails fall off when the iron sheet 
is introduced ? 

3. Will the nails fall off if a copper or alu- 
minium sheet were introduced instead of the 
iron sheet ? 

Explanation 

1 . The magnetic lines of force pass through the 
plastic sheet. Hence the nails are attracted 
owing to magnetic induction. 

2. When an iron sheet is introduced the 
magnetic lines of force get concentrated in 




sheet though there is no direct connection 
between the magnet and the nails. 

2. Now, raise the magnet a little and intro- 
duce the iron sheet over the plastic sheet. 
You will observe that the nails fall off the 
sheet. 

Students to enquire tmmmmmMMm 

1 . Why do the nails stick to the plastic sheet 
even though the nails do not touch the 


magnet 


? 


the iron sheet and do not pass through the 
iron sheet. Hence the magnet cannot induce 
magnetism in the nails. So the nails fall off. 
This is called magnetic screening. 

3. If a copper or aluminium sheet were intro- 
duced, then the nails would not fall off. 
This is due to the fact that aluminium and 
copper are not magnetic substances. Non- 
magnetic substances cannot screen the lines 
of force. 




Quest 83 









Rainbow on paper 



*, * ■ P • * * > ■ P •» m M M ■ ■ ■ A 

9 B*9»mmmT* r P F f W* l 

mKmmrmmr m + *upmvm*w* 



Materials 

1 . Water in a container 

2. A minor 

3. A white sheet of paper 

What to do ? 

1 . Place the mirror in the container in such a 
way that the major part of the mirror is 
below the water. 

2. Do the experiment in sunlight. Try to reflect 
the sunlight on the paper with the mirror. 



surface of water ? 

2. When the sunlight falls on the water, what 
happens to the light ? 

3. How is the rainbow created on the paper ? 

Explanation 

1. If the sunlight falls directly on the minor, 
the light gets reflected. 

2. When the light falls on the surface of the 
water, it gets refracted. 

t 

3. Sunlight is composed of seven colours. When 



You have to keep the paper fixed and the 
reflection of sunlight should take place from 
that part of the minor which is below the 
water. You have to adjust the mirror, if 
necessary. 

Students to enquire 
1 . What happens if the sunlight falls on that 
portion of the mirror which is above the 


the sunlight falls on the surface of the water, 
the seven colours are refracted at different 
angles and are separated. When these colours 
fall on the submerged portion of the mirror, 
they get reflected. When these different 
coloured lights come out of the water, they 
form the rainbow on the paper. 



Quest 84 








Materials 

1. A bowl 

2. A coin 

3. Water 
What to do ? 

1 . Place a coin at the centre of the bowl. 

2. Place your eyes in such a positon that the 









4. You will observe that the coin suddenly 
comes into your sight though you have not 

moved your eyes (Fig. B). 

• % 

Students to enquire 

1 . Why could you not see the coin when you 
placed your eyes as at point A? 

2. Why does the coin reappear again? 

Explanation 
1. We see an object 
^ when light from 

that object comes to 
V our eyes. Because 

/pj. the rim of the bowl 

^ ? I obstructed the light 

J from reaching our 

eyes we could not 






* 


coin just vanishes from your sight. Keep 
your eyes fixed there (Fig. A). 

3. Now let your friend gently pour water into 
the bowl without displacing the coin. 


P sec the coin. 

2. When light rays 
pass from one me- 
dium to another, 
they bend from their 
original path. This 
- — - bending of light is 

called refraction. 
When light passes 
from a denser to a 
rarer medium it 
bends away from 

the normal to the 

water surface. Due to this an object in water 
is seen raised. This refraction of light from 
the water surface made the coin visible. 


Quest 85 




1. A comb. 

What to do ? mmmmfMWMmmBm. 

1 . Hold the comb with your left hand. Draw 
your finger over the teeth of the comb from 
one end to the other. Listen to the sound. 

2. Now, keep the comb j 
vertically on the table. 

Repeat the operation. 

You will observe that the 
intensity of sound has 
increased . 

3. Keep the comb on other 
objects and repeat the 

experiment. 

Students to enquire 

1. Why does the comb make / 

a sound when you draw / j 

your finger over the /wt 

comb ? 

2. Why does the sound in- 
crease when you keep the 
comb on the table and 
draw your finger over the 
comb ? 

3. Where is this method of 
increasing the sound 

utilised ? I — 

Explanation 

1 . When you draw your finger over the comb, 
the teeth start vibrating. A vibrating body 






out 


When 


held in the air, only the teeth and the comb 
start vibrating. But when the comb is held 
on the table a forced vibration is set on the 
table also. Owing to the combination of 
these two vibrations of the comb and the 
table the intensity of sound is increased. 


m 


u 


c. 




\ 


3. In different instruments like the sitar, the 
sarod, the violin, the tanpura there is a hollow 
box made of wood or other material. This 
increases the sound when the instrument is 
played. 



Quest 86 





Materials 

1 . A narrow-necked empty bottle 

2. Water 

3. A scale or a key 

What to do ? 

1 . Fill the bottle with water keeping a little 
gap at the top. Blow air at the mouth and 
try to bring out sound like a flute. Notice 
the pitch of the sound. 

2. Reduce the water of the bottle in steps and 
repeat the activity. Notice the pitch of the 
sound as the water level goes down. 


bottle as the water level goes down ? 

2. Why does it change so ? 

3. What happens to the pitch of the struck 
bottle as the water level goes down ? 

4. Why does it change so ? 

1 . As the water level goes down the pitch of 
the whistle sound also goes down. 

2. The length of the vibrating air column in- 
creases as the water level goes down. The 
pitch of vibrating air column is inversely 
proportional to its length. Hence, the more 



3. Now, fill the bottle again with water and 
strike the bottle with the key or the scale. 
Notice the pitch. 

4. Reduce the quantity of water in steps and 
repeat the activity. Notice the pitch of sound 
as the water level goes down. 

Students to enquire 

1 . What happens to the pitch of the whistling 


the length the less is the pitch. 

3. If you strike the bottle as the water level 
goes down, the pitch of sound goes on in- 
creasing. 

4. As you reduce the water , the total mass of 
the bottle lessens. The pitch of sound in- 
creases with the reduction of the mass of 
the bottle. 



Quest 87 











Dancing paper strips 



Materials 

1. A plastic comb 

2. Some small pieces of paper 
What to do ? WMMMBmm 

1. Take the dry comb and coml 

two or three times. 

2. Then, brine the comb near 


wmmM 



are 


attracted by the comb. 



What 






a 


a 


Explanation 

1. When the hair is combed 


When 


near 


minium foil or thread, the 


Opposite 


tract 


the pieces of paper, thread or aluminium 


a 


^ ^ ' 

sv 


see 


the pieces of paper are attracted. 


Students to enquire 

1 . Why are the pieces of paper attracted by the 

comb ? 

2. If the comb is negatively charged, what is 
the charge formed in the pieces of paper or 
aluminium foil ? 




foil. 


If the comb is negatively charged the pieces 
of paper or aluminium foil or thread will be 

positively charged. 


>lastic, glass or ebonite rods 
flannel or silk they will be 


charged. 



Quest 88 




Materials 


mmm? 


1 . A plastic comb 



2. An empty powder can with a hole at the 
bottom 

3. A bucket 
What to do ? 






1 . Fill the can with water and let the water fall 
into the bucket. 


wmlf on your hair and hold the 
comb near the flowing water. 

3. You will observe that the water stream will 
be attracted towards the comb. 

4 . If the comb or the hair is wet, the experi- 



ment will not work. 

Students to enquire 

1 . Why does the water stream tiend towards 
the comb ? 

2. Will a thick stream of water bend ? If not, 
why ? 

3. Why does not the stream bend if the comb 


is wet ? 




mmmm 


4. What else can 1 

€ 

sides a comb ? 

Explanation 

1. When the hair is combed 
the comb becomes chatged. 
When the charged comb is 
brought near the water 
stream, the stream gets an 
opposite charge by induc- 
tion. This is why the stream 
gets attracted by the comb. 

2. As the attractive force of 
static electricity is very 
feeble it cannot bend a thick 
stream. 

3. If the comb or the hair is 
wet, the comb will not be 
charged. This is the reason 
why in the rainy season the 
static electricity experiments 


performed 


4. 


balloon 


plastic cards 


wool 


flannel 



Quest 89 




Materials 
1 1 Sugar and p 
2. A dry comb 
What to do 





mmmmm 


2. Why are 


powder 




3. In what other ways can you separate the 

pepper powder from the sugar ? 

* . . . . 

Explanation 




1. 


powder 


When 


2 . 


ifiiA r i * 

it on a piece of paper. 

Comb your hair and bring the comb near the 

mixture of sugar and pepper powder. You 
will observe that the pepper powder will be 
attracted very easily by the comb. Collect 
the powder on a piece of paper by striking 

the comb on the table. 


When you 


vnui^vui «r ^ % * 

near the mixture, the opposite charge is in- 

l thp 


powder 


granules of pepper are attracted by the comb 


The 


chemical composition 


tUllIU UVVUMtyv 

3 if we mix sugar and pepper powder in water. 



3. You can seperate all the pepper powder by 
repeating the operation several times. 

4. You will observe that if you do not comb 

« Ml a . U ^ 


sugar will be dissolved in water, but pepper 

^ M V/Ml 


powder 


powder 


granules 


0 

attracted by the comb. 

Students to enquire HiHIK 

1 . Why are the pepper granules attracted when 

you use the comb after using it on hair ? 


separate it from the sugar solution. If you 
now evaporate the water of the sugar solu- 
tion, sugar crystals will be left. In this way 
also you can separate the two compounds 
nf thr. suear and pepper powder mixture. 




Quest 90 




W&mm. 



ill 


Materials 
1. Three balloons 
What to do ? 

1 . Rub the balloons on your shirt. 

2. Take the balloons close to a dry wall. You 
will observe that the balloons get stuck to 
the wall. 



mmmsmm 


Students to enquire 
1 . Why are the balloons rubbed on the shirt ? 


2. Why do the balloons stick to the wall ? 

3. Why is a dry wall required for the experi- 
ment ? 

4. Will the balloons stick for ever to the wall? 



wmm 




Explanation 

1 . The balloons are rubbed on the shirt to get 
them charged. 

2. When the balloons are rubbed, some elec- 
trons pass from the shirt to the balloon. 
Normally there are equal numbers of pro- 
tons and electrons in an uncharged object. 
Hence the object is electrically neutral. But, 
when excess of electron accumulates on the 

balloon it gets negatively 
charged. When negatively 
charged balloons are brought 
near the wall, the electrons 
on the surface of the wall are 
repelled. Thereby the wall 
gets positively charged. Un- 
like charges attract each 
other. Hence the balloons get 
stuck to the wall. 

3. If the wall is wet, it will lose 
charge very quickly. The 
same thing happens if the air 
is very humid. Hence the wall 
has to be dry. 

4. If the balloons arc kept on 
the wall for long, then the 
excess electrons from the 
balloons will flow back to 
the wall. The balloons will 

become neutral and will fall. Moreover, if 
the balloons are kept for long, they will 
gradually collapse. 


Quest 91 


t 


Materials 



that the comb will attract the scale and it 


wooden 


tumble down 


2. A dry comb 


Students to enquire 



What 



Why 


the 


Explanation 



such a way that the 6" mark is in line with 


When you rub your hair, 



the edge of the table and the scale just 


When the charged comb 


balances. 


opposite 


2. Brush your dry hair with the comb and 


duced in the scale and the scale is attracted 


bring the comb below that end of the scale 


by the comb.The scale loses its balance and 


MW 

which is outside the table. You will observe 


down 



Quest 92 



Material 





1 . A piece of asbestos or a metal plate 

2. An ink dropper 

3. Aluminium dust 

4. Iodine crystals 
What to do ? 

1 . T ake one teaspoonful of iodine crystals and 



same 


powder 


4. After a little while you will observe that 
smoke starts coming out and then the re- 
agents catch fire. 





2. Mix the powders on the plate and give the 
mixture the shape of a cone and make a 
depression on the top. 

3. With the help of a dropper, let one or two 
drops of water fall on the top of the heap. 


Students to enquire 

1. Why do you have to grind the iodine 
crystals ? 

2. Why do the reagents have to be dry ? 

3. Will the reagents react without water ? 

4. Where in our daily life does water produce 

a reaction like this ? 

Explanation 

1. If you crush the iodine 
crystals, the contact area 
between the two reagents 
increases. This increases 
the rate of reaction. 

2. If the reagents are wet, 
the heat created by the 
reaction will be absorbed 
by water and the rate of 
reaction will be lowered. 
For this reason the re- 
agents will not catch fire. 

3. Water acts as a catalyst. 
Reaction will not start 
without water. Water first 
helps in starting the reac- 
tion in a few molecules. 

% 

This causes generation of heat which in turn 
helps in further reaction. 

4. In our daily life, rust is formed because of 
similar reaction aided by water. 



Quest 93 




Materials 






ink 


500 c.c. round-bottomed 


2 . A two-holcd rubber stopper with a long 
glass tube in one hole and an ink dropper 

in the other hole 

3. A test tube with a rubber stopper through 
which there is a bent tube 


4. When will the water start entering the flask? 

5 . Why does the water enter the flask like a 


fountain ? 


500 


(NH 


4 ' 


carbonate 


lamp 


Explanation 

1 . When ammonium chloride and calcium car- 
bonate are mixed together and heated, am- 
monia gas is formed. The following reaction 

takes place : 

2NH 4 C1 + CaC0 3 = 2NH 3 + CaCl 2 + H 2 0 

2. Density of ammonia ‘ " “ 



“MNB 


What 

1. Take NH 4 C1 and CaC0 3 in the test tube. 
Close the mouth of the test tube with the 

stopper and heat it. 

2. Fill the ink dropper with water. 

3 . Invert the flask and introduce the bent tube 

inside the flask. 

You will get the pungent smell of ammonia. 
Continue heating the flask for a few seconds 

Then close the mouth of the flask with 


4 



the rubber stopper having the long tube and will rise up in the air. Hence the flask has 

•i « . 1 J ^ rl a 


ink 


5 


A A 

ig the flask inverted 


to be held upside down. 

3 . Ammonia is very soluble in water. So, when 

sprinkled inside the flask, ammonia 


of the long tube in the beaker of water. . ... m 

Sprinkle a little water inside the flask by ga s gets dissolved in water and a partial 

vacuum is created inside the flask. 


will observe that water will enter the flask 4. The 


like a fountain. 

Students to enquire 

1 . What happens when the two 
mixed together and heated ? 

2. Why do you have to inver 

collect the gas ? 

3 What is the function of water 1 





a partial vacuum is created inside the flask. 
5. After the partial vacuum is created inside 
the flask atmospheric pressure working on 
the water in the beaker forces the water 
inside flask to gush like a fountain. The 
narrower the end of the tube the better will 

be the fountain effect. 



Quest 94 
















mm * 


ii 


mmrnmmmmm 


m 




mm 


Cooling by shaking 



^^SSSKSS 


Materials ^OM M MMM 




1 . A conical flask with a rubber stopper 

2. Barium hydroxide (Ba(OH) 2 ) and ammo- 
nium thiocyanide (NH 4 SCN) 

What to do ? 

1 . Take one teaspoonful of each chemical in 
the flask. 


the two chemicals ? 

Why should the wooden board 


Explanation 





When two compounds react with each other 
either heat is given away or taken in. If heat 
is given away, it is called exothermic reac- 
tion, and if heat is taken in, it is called 



Close the mouth with the stopper and shake 
the flask for some time. After shaking it for 
some time you will observe that the flask is 
getting cooler. 

If you keep the flask on a wet wooden board, 
it will stick to the board. 


Students to enquire 

1 . What kind of reaction takes place between 


endothermic reaction. The above reaction is 
endothermic. Hence, it takes heat away from 
its environment i.e., from the flask. Hence 
the flask cools down. 

2. When the flask gets cooler, it cools the 
water of the board to ice. Because of this, 
the flask gets stuck to the board. 


Quest 95 










mm 








*tppp 


m m m ^ 

* 

Which gases are formed 


sm 







sek 


ipp* 8 * 


Explanation : 


1 . Zinc granules, calcium carbonate (CaC0 3 ) 

2. Potassium chlorate (KC10 3 ), dilute hydro- 
chloric acid (HC1) 

4 • • • • I. 

% _ • 

3. A spirit lamp or a heater 

4. Three test tubes, a match box and a little 
ash. 

what to do ? mmmmm wmmmmmm 

9 • 

• 9 * « 

1 . Take Zinc granules in the first test tube and 

add 3-4c.c. HC1 to that. Close the mouth I 

* ■ 

of the test tube with your thumb. After 
sometime take away your thumb and 
bring a lighted matchstick near the mouth 
of the test tube. You will observe that 
there will be a small explosion. 

2. Take CaC0 3 (pieces of marble) in the 
second test tube and add 3-4 c.c. of HC1. 
Introduce a burning stick into the test 
tube. You will observe that the fire will 
extinguish. 

3. Take a little KC10 3 mixed with ash in 
the third lest tube. Heat the test tube. 
Light a wooden stick and put it off and 

then immediately introduce the stick into | 

the test tube. You will observe that the stick 

will again light up. 

Students to enquire 


.SV.'.V.V. 




Which gas was formed in the first test tube 
and what is its characteristics ? 

9 

Which gas was formed in the second test 
tube and what is its characteristics ? 
Which gas was formed in the third test tube 
and how is it different from the other two ? 


Zn 


formed 

ZnCl-. 


highly inflammable. So, when the lighted 

• « 

matchstick is brought near it a small ex- 

• * ft 

ft .< • ■ 

• • 

plosion takes place. 


formed 


This 


combustion. It is denser than air and is 



colourless. CaC0 3 + 2HC1 = CaCl 2 +C0 2 
+H 2 0. 

3. Oxygen is formed in the third test tube. 
Oxygen helps in combustion but is not itself 
inflammable like hydrogen. Hence, when a 
lighted stick that has just been extinguished 
is brought in contact with oxygen the stick 
is ignited again. Oxygen makes up about 
20% of atmospheric air. 


Quest 96 




Materials 

1 . Sodium peroxide (N^Oj) 

2. Lycopodium powder 

3. An ice cube 

w 

4. A metal sheet 
What to do ? 

1 . Take two teaspoonful of lycopodium pow- 
der in a heap on the metal plate, give the 

heap the shape of a cone. 

% 

2. Put half a teaspoonful of sodium peroxide 




« . r . 

5. Is the reaction that takes place exothermic 

* * * • # * . v • 
*••• • , « » * 

or endothermic ? 

• * * • • _ • 

6 . What else can you use in lieu of lycopodium 

* 

powder ? 

Explanation 

1 . When ice is kept at room temperature, it will 

• • 

melt into water. 

• " % 

• * 

2. NajOj + H 2 0 -> NaOH + O n + energy 
(heat). 

3. Sodium peroxide supplies oxygen for spon- 




on the top of the heap. Put the piece of ice 
on it. 

3. Stand a little distance off. You will observe 
that the lycopodium powder will be ignited. 

Students to enquire 

1 . What happens to the ice when it is kept at 
room temperature ? 

2. What reaction occurs between sodium per- 
oxide and water ? 

3. What is the function of sodium peroxide ? 

4. Why is the lycopodium powder ignited ? 


tancous combustion. 

4 . Reaction takes place between sodium per- 
oxide and water from melted ice nascenf 
oxygen is formed, nascent oxygen is very 
reactive. It oxidizes the carbon in the lyco- 
podium powder and spontaneous combus- 
tion starts. This sets the lycopodium powder 
into flame. 

* 

5. The reaction is exothermic. 

6. You can use tissue paper, fine saw dust, or 
starch instead of lycopodium powder. 



Quest 97 







■■■■ft 



Materials 

1 . Calcium hydroxide (Ca(OH) 2 ) 

2. A test tube 

3. A drinking straw 

4. Filter paper 

What to do ? 

1 . Fill half the test tube with calcium hydrox- 
ide solution. If Ca(OH) 2 is available in 
powder form, then pre- 
pare a saturated solu- 
tion of Ca(OH) 2 in 
warm water, keep the 
solution for some time 

* 

and then filter out the 
clear water from the 
top. 

2. Blow air into the solu- 
tion through the straw. 

You will observe that 
the solution starts be- 
coming cloudy due to 
the formation of a 
white precipitate. 

3 . If you continue blow- 
ing air into the cloudy 
liquid, it will gradually 
clear up. 

Students to enquire 

1. Why does the precipitation take place ? 

2. Normally, when does precipitation occur ? 

3. Why docs the precipitate disappear ? 


dioxide (C0 2 ) is added to the solution. CO z 
reacts with calcium hydroxide (Ca(OH) 2 ) 
and calcium carbonate is formed which is 
insoluble in water. Hence a precipitate is 
formed. C0 2 + Ca(OH) 2 = CaC0 3 + H 2 0. 

2. When the solubility of a substance in a 
liquid is less, precipitation takes place. 

3 . When you continue blowing more C0 2 into 




1. When you blow into the solution carbon 


the precipitate calcium bicarbonate 
(Ca(HC0 3 ) 2 ) is formed. CaC0 3 + H 2 0 + 
C0 2 = Ca(HC0 3 ) 2 , Calcium bicarbonate is 
soluble in water. Hence the precipitate dis- 
appears. 



Quest 98 



Materials 



1. A 250 c.c. conical flask with rubber 


stopper 

2. 5 gms. potassium or sodium hydroxide 

3. 3 gm. glucose or dextrose 

4. Methylene blue 


What to do ? 



1 . Dissolve 5 gms. potassium hydroxide and 3 


2. Why does the colour disappear on keeping 
still ? 

3. Can the experiment be conducted 
repeatedly ? 

4. What is the nature of the chemical reaction 


which is taking place ? 



1 . In an alkaline solution of a reducing sugar, 


gms. glucose in water in the flask. Add a | methylene blue is reduced to a colourless 



few drops of methylene blue in the liquid. 
A colourless liquid will be formed. 

2. Close the mouth of the the flask by the 
stopper and shake the flask. You will ob- 
serve that the colourless liquid be changed 
to a deep blue colour. 

3. Keep the flask on the table for some time. 
The liquid will again become colourless. 

4. Shake the flask and the deep blue colour 
will appear again. Keep the flask still for 
some time and the colour will disappear. 


Students to enquire 


■HHB 


1 . Why does the deep blue colour appear on 


shaking ? 


compound. On shaking, the oxygen in the 
air inside the flask dissolves in the liquid 
and the colourless solution is rcoxidized into 
the blue dye. The solution turns deep blue. 

2. When the flask is left still for some time, 
the oxygen goes out of the solution. The 
solution is reduced again and its colour 
gradually fades away. 

3. The experiment can be repeated several 
times but after a few days the solution will 
turn yellow and then brown. 

4. It is a reversible oxidation-reduction 
reaction. 



Quest 99 











* • * 4 • 

Materials 

1 . Six test tubes on a stand 

. ♦ * 

2. Dilute hydrochloric acid 

3. Liquid sodium hydroxide 

4. Phcnolphthalcin 

5. A drinking straw 

fi A glass and an ink drODi 



•I 


What to do ? 

1 . Number the test tube 1 to 6 from the left. 

2. Place three drops of phenolphthalein indi- 
cator in the test tubes 2 and 4. Take one 
drop of sodium hydroxide in the test tube 


3. 


4 


6 and in the glass. Take three drops of 
hydrochloric acid in the test tube 5. 

Fill the glass three-fourths full with water. 
Now fill the test tubes 1 , 2, 3 and 4 half full 
with water from the glass. You will observe 
that the liquids in the test tubes 2 and 4 turn 
pink. 

Now, pour the liquids from the four test 
tubes into the glass. 

5. Now fill the test tubes from 1-5 with the 
liquid from the glass. Except in the test-tube 
5, liquids in all other test tubes will show 
pink colour. 

6. Now pour the liquids from all the test tubes 
into the glass and fill up again all the six 
test tubes. You will observe that only the 


Students to enquire 


pinkish 



Why 


pink 


Why does the liquid 

colourless ? 

Why does the liquid 
pink colour ? 



\ 

Explanation 
1 . Phenolphthalein is an indicator for alkaline 


pink 


There 


phthalein in test tubes 2 and 4 and sodium 


When water is 


in the glass an alkaline solution is created. 
So, when the alkaline solution is added to 
the test tubes 2 and 4 the phenolphthalein 



2. There is hydrochloric acid in test tube 5. 
When the pink alkaline solution is poured 
into the test tubes 1 to 5, the alkaline so- 
lution is neutralised by the hydrochloric acid 
in the test tube 5. Hence the liquid in the test 
tube 5 becomes colourless. Phenolphthalein 
is colourless in neutral solution. 

3. As the test tube 6 contains sodium hydrox- 
ide, the solution becomes alkaline again and 
phenolphthalein in alkaline solution turns 
pink in colour. 



Quest 100 




• * 

Materials 

S a 9 f 

9 a 

1 . A stick one metre long, 

2. Filter paper 

• 9 # § » • 

3. Iodine crystals 

4. Concentrated ammonia 

5. A 100 c.c. beaker 
What to do ? 

1. Dissolve iodine crystals in concentrated 

ammonia in the beaker. Be sure that all the 
crystals are dissolved. Stir the liquid with 
a glass rod. ' 




2. Let the beaker stand in the air till a brown 
residue is left in the beaker. 

3. When the residue is still wet, scratch out the 
residue with the help of the stirrer on the 
filter paper. Allow it to dry. It will take 




» • .. # " m 4 

about an hour to dry. The more dry the 
residue the better the experiment. 

4. Stand about a metre apart from the filter 
paper and then tap the dried up crystals with 
the help of the long stick. Tap only a few 
crystals at a time because the crystals are 
highly explosive. 

* k 

Students to enquire 

1. Which compound is formed when iodine 
crystals are dissolved in concentrated 
ammonia ? 

2. Why is the compound so explosive ? 

3. Why do we hear a loud report when there 
is an explosion ? 

4. Is oxygen always needed whenever there is 
an explosion ? 

Explanation 

1 . When iodine crystals are dissolved in con- 
centrated ammonia, nitrogen tri-iodide is 
formed. 

2. Wet nitrogen tri-iodide is not that danger- 
ous. But when much of ammonia gets evapo- 
rated, dry nitrogen tri-iodide becomes un- 
stable. When it is tapped by the stick, it 
explodes. If the amount of nitrogen 
tri-iodide is less, then there is less danger of 
a big explosion. 

3 . As a large amount of gas is produced during 
an explosion, there is a loud sound. 

4. When there is a sudden release of a large 
volume of gas, there is a loud sound. As for 
example, when a balloon is pricked the air 
under pressure is suddenly released. Hence, 
there is a loud sound. Oxygen is not 
indispensible for an explosion to occur. 


Quest 101 








-mmi W& 




w? 








Acid heats up 




fP^' 


p 




Materials 


#p-£ 




1. Potassium chlorate (KQO 3 ) 


2. Potassium permanganate ( KMn0 4 ) 


3. Ground sugar 


4. Concentrated sulphuric acid (H 2 S0 4 ) 


5. An ink dropper, a watch glass 


What 




WMi 



1 . 


potassium 


potassium permanganate 


2. Pour a drop of sulphuric acid 


on the heap of the mixture with 


the help of the dropper. Drop 


the acid from a distance. It is 


advisable to conduct the experi- 


ment in the open. Take great 


care while handling the acid. 


You will observe that the mix- 


ture will catch fire and a large 


volume of gas will be produced 


If a little barium, strontium. 


copper or sodium salt is added 


to the mixture, then you will 


observe the magic of colours. 


Students to enquire 



1 . What is the source of energy 


for the generation of heat ? 


2. Which one among the 


kinds of powder was burning 7 


3. Where did the heat come from 


for the spontaneous combustion ? 


Which gases are formed 


1 . The chemical energy stored in the chemical 


compound 


2. 


powder that was burning 


3. Sulphuric acid supplied the first heat to 


potassium 


The heat helped 


potassium chlorate. This 


gen oxidized the sugar and so much heat 


was liberated that the sugar caught fire. 


Potassium chlorate (KC10 3 ) supplied oxy- 

_ . - 






4 






gen for combustion. 


5 . Carbon dioxide and water vapour are formed 


by combustion. 



Quest 102 








Invisible writing 




mm 








mm*®** 




1 . Cobalt chloride crystals 

2. Ammonium chloride 

3. Distilled water 

4. A little cotton 

5. A piece of pink paper 

% 

6. A candle, a match box and a wooden stick 

What to do ? gHB I B855S 

1. Mix 1 V 2 gms. ammonium chloride with 
2V 2 gms. of cobalt chloride. Add 50 c.c. of 
distilled water and make a solution. You 
will observe that the colour of the solution 


will be deep pink. 


5. You will observe that the word “chemistry” 
will reappear in bright blue colour. When 
the paper cools down the inscription disap- 
pears again. 

Students to enquire WMMMmmim 

1 . When you write on the pink paper, why does 
it disappear when the paper dries up ? 

2. Why does the inscription reappear on 
healing ? 

3. Why does the inscription disappear again 
on cooling ? 

Explanation VmHmmmmmmmmmm 

1. As the colour of both the paper and the 



Wrap a little cotton at one end of the stick. 
Dip the cotton in the solution and write the 
word “chemistry” on the pink paper. 
Allow the paper to dry, you will observe 
that the inscription will gradually disap- 
pear. 

Now, light the candle and bring it near the 
paper so that the written word may get the 
heat from the flame of the candle. 


solution is pink, inscription disappears. 

2. When cobalt chloride goes into solution, a 
complex compound is formed which releases 
water on heating. In this condition the colour 
of the compound is blue. 

3. When the paper cools, it absorbs moisture 
from the atmosphere and the pink colour of 
the solution reappears. Hence inscription 
disappears again. 



Quest 103 







Materials 


1. 100 grams sugar 






3. A small glass tumbler 

4. Concentrated sulphuric acid 

What to do ? 

% 

1 . Fill half the glass and the jar with sugar. 

2. Pour some water in the glass and about 
40 c.c. of concentrated sulphuric acid in the 
jar. 

3. Wait for some time. You will 
observe that in the glass the 
sugar will dissolve in water. But 
in the jar a blank voluminous 
substance will be produced 
which will gradually rise up in 
the jar and come out of it. 

Students to enquire PM* 

1 . What is the black substance in 
the jar ? 

2. What is the difference between 
the processes that take place in 
the glass and in the jar ? 

3. What difference did you 
observe in the experiment be- 
tween the physical and chemi- 
cal processes ? 

4. From which container can you 

separate the sugar and how ? 

5. What happens in the jar ? 

Explanation 

1 . The black substance in the jar is charcoal. 

2. The process that takes place in the glass is 


mm am 




m&m 


a physical change. But a chemical change 
takes place in the jar., 

w 

3. In a physical change you can bring back the 
original substances. But in a chemical change 
original substances cannot be brought back 
easily. 

4. You cap separate the sugar from the sugar 
solution in the glass by the evaporation of 
water by heating. 



5 . The sulphuric acid reacts with sugar chemi- 
cally. As a result, charcoal, sulphur dioxide 
gas and water vapour are formed. These 
gases cause the expansion of charcoal into 
a voluminous mass. 


Quest 104